Wallboard is the standard material used to erect interior walls in the construction of homes and buildings in the USA, Canada, and some other countries. Generally, wallboard is installed in large panels that are nailed and glued to the wall studs and fitted together until the wall section is covered. The joints where sections of the board are joined are covered with joint compound, then with tape, and then the taped joints and all nails and screws are covered with joint compound. Upon hardening, the joint compound is sanded smooth so that it is imperceptible under paint or other wallcovering.
Joint compounds generally contain a resinous binder, limestone, clay, mica, stabilizer, lubricant, and thickener as the principle ingredients that are mixed with water to form a workable mass that is applied to the wall by troweling. Over the past several years, many of the general components in joint compounds have undergone refinement for various reasons. For example, asbestos is no longer used in joint compounds for health reasons and different types of clays such as attapulgite clay, kaolin clay, bentunite and montmorillonite have been tried in different formulations.
The stabilizers and thickeners in the joint compounds have also undergone changes in order to try to improve the joint compounds. The typical water soluble polymers used to thicken and stabilize joint compounds are cellulose ethers, e.g., methylhydroxypropylcellulose (MHPC), hydroxyethylcellulose (HEC), and derivatives thereof. The cellulosic thickeners are chosen so as to impart excellent working properties to joint compounds. However, each of the common cellulosic thickeners often imparts one or more properties to a joint compound that need to be improved. For example, less than satisfactory sag resistance (so-called "body") occurs with MHPC; gelation that results in poor workability may occur with HEC. These undesirable characteristics of joint compounds are well known in the drywall finishing industry.
Guar has also been used to thicken joint compounds in the prior art with limited success. Although guar or substituted guars such as hydroxypropylguar (HPG) have been used, unsubstituted (underivatized) guars usually thickened excessively after relatively short periods of aging; excessive gelation is also common. When these guars are used, the joint compounds are at first acceptable; however, upon standing, the shelf-life is unacceptably short. It has been found that one factor that makes these guars unacceptable is the presence of borate.
Guars (underivatized and derivatized) are typically treated with borax in order to facilitate purification during their manufacturing process. Even small levels of borate in amounts above about 200 parts per million (ppm) will cause unacceptable thickening and syneresis in joint compounds over a period of time from several hours to days. As a matter of fact, borate at this level is intolerable in a joint compound formulation using guar as a thickener regardless of the source of the borate. Therefore, if the borate has not been removed from the guar or the formulation, a complexing agent must be added to the system to chemically complex the borate so as to render it inactive. Since borate is a naturally occurring element, it may be present in trace amounts from any of the ingredients that are found in nature.
U.S. Pat. No. 4,558,079 is an example of a patent that discloses the use of a guar derivative in a tape joint compound composition suitable for use with wallboard where at least one hydroxypropylhydroxyethylcellulose (HPHEC) having specified substitution levels is employed as the water retention aid and thickener. Blends of HPHEC are disclosed with (a) at least one cellulose ether, such as, hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), methylcellulose (MC), methylhydroxyethyl cellulose (MHEC), and methylhydroxypropylcellulose (MHPC) or (b) a hydroxyalkylguar, such as hydroxypropylguar and hydroxyethylguar. This patent does not address the problem of borate contaminated guar.
No prior art was found that provides an alternative composition to improve the properties of tape joint compounds by providing an acceptable substitute, in toto or in part, for the cellulose ethers that are customarily used to thicken and modify the rheological properties of the joint compound using a guar derivative in the absence of borate.